cubicweb #4143913 make session handling compatible with the requirements of an integration under pyramid [open]

IOW we want to be able to delegate most session.data (and tracking as a whole) to pyramid or another web broker.

http://docs.pylonsproject.org/projects/pyramid/en/latest/narr/sessions.html#sessions-chapter

prioritynormal
typeenhancement
done in<not specified>
load1.000
load left1.000
closed by<not specified>